Rewritable nano-surface organic electrical bistable devices

ABSTRACT

A bistable electrical device that is convertible between a low resistance state and a high resistance state. The device includes at least one layer of organic low conductivity material that is sandwiched between two electrodes. A buffer layer is located between the organic layer and at least one of the electrodes. The buffer layer includes particles in the form of flakes or dots of a low conducting material or insulating material that are present in a sufficient amount to only partially cover the electrode surface. The presence of the buffer layer controls metal migration into the organic layer when voltage pulses are applied between the electrodes to convert the device back and forth between the low and high resistance states.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national phase of International Application No.PCT/US04/02932, filed Feb. 2, 2004, and claiming priority of U.S.Provisional Application No. 60/444,748, filed Feb. 3, 2003, the entirecontents of which are incorporated herein by reference.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of the Air ForceOffice of Scientific Research Grant No. F49620-01-1-0427, and the Officeof Naval Research (ONR) Grant No. N00014-01-1-0855.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electronic devices thatutilize elements that exhibit bistable electrical behavior. Moreparticularly, the present invention is directed to organic semiconductordevices including electrically programmable nonvolatile memory devicesand switches.

2. Description of Related Art

The publications and other reference materials referred to herein todescribe the background of the invention and to provide additionaldetails regarding its practice are hereby incorporated by reference. Forconvenience, the reference materials are numerically referenced andidentified in the appended bibliography.

Many electronic memory and switching devices typically employ some typeof bistable element that can be converted between a high impedance state(off-state) and a low impedance state (on-state) by applying anelectrical voltage or other type of writing input to the device. Thisthreshold switching and memory phenomena have been demonstrated in bothorganic and inorganic thin-film semiconductor materials. For example,this phenomenon has been observed in thin films of amorphouschalcogenide semiconductor (1), amorphous silicon (2), organic material(3) and ZnSe—Ge heterostructures (4).

The above materials have been proposed as potential candidates fornonvolatile memories. The mechanism of electrical bistability has beenattributed to processes such as field and impact ionization of traps,whereas in chalcogenide semiconductors they involve amorphous tocrystalline phase changes. Analogous memory effects in the leakagecurrent of ferroelectric BaTiO₃ or (Pb_(1-y)La_(y))(Zr_(1-x))O₃-basedheterostructures have also been reported and discussed in terms of bandbending due to spontaneous polarization switching. Electrical switchingand memory phenomena have also been observed in organic charge transfercomplexes such as Cu-TCNQ[5,6].

A number of organic functional materials have attracted more and moreattention in recent years due to their potential use in field-effecttransistors (7), lasers (8), memories (9,10) and light emitting diodesand triodes (11,15). Electroluminescent polymers are one of the organicfunctional materials that have been investigated for use in displayapplications. In addition to display applications, electroluminescentpolymers have been doped with high dipole moment molecules in order toobtain a memory effect (12). This memory effect is observed when dipolegroups attached to side chain of the polymer rotate due to applicationof a threshold bias voltage. Unfortunately, rotation of the dipolegroups takes a relatively long time. Also, doping of the polymer reducesthe electroluminescence of the doped polymer.

Electronic addressing or logic devices are presently made from inorganicmaterials, such as crystalline silicon. Although these inorganic deviceshave been technically and commercially successful, they have a number ofdrawbacks including complex architecture and high fabrication costs. Inthe case of volatile semiconductor memory devices, the circuitry mustconstantly be supplied with a current in order to maintain the storedinformation. This results in heating and high power consumption.Non-volatile semiconductor devices avoid this problem. However, theyhave the disadvantage of reduced data storage capability as a result ofhigher complexity in the circuit design, and hence higher cost.

A number of different architectures have been implemented for memorychips based on semiconductor material. These structures reflect atendency to specialization with regard to different tasks. Matrixaddressing of memory location in a plane is a simple and effective wayof achieving a large number of accessible memory locations whileutilizing a reasonable number of lines for electrical addressing. In asquare grid with n lines in each direction the number of memorylocations is n². This is the basic principle, which at present isimplemented in a number of solid-state semiconductor memories. In thesetypes of systems, each memory location must have a dedicated electroniccircuit that communicates to the outside. Such communication isaccomplished via the grid intersection point as well as a volatile ornon-volatile memory element which typically is a charge storage unit.Organic memory in this type of matrix format has been demonstratedbefore by using an organic charge transfer complex. However such organicmemories require transistor switches to address each memory elementleading to a very complex device structure.

Organic Electrical Bistable Devices (OBD's) have been proposed in thepast where a metal layer is sandwiched between two organic layers. Thissandwich structure is used as an active medium that is interposedbetween two electrodes. Controllable memory performance has beenobtained using this type of configuration. A positive voltage pulse isused for writing, while a reversed bias is used for erasing. Theshortcoming of this kind of memory device is that erasure must beperformed by applying a reversed bias. In an x-y electrical-addressablememory array application, a diode must be series connected with eachmemory cell to prevent the so-called “sneak current”. In this type ofapplication, it is difficult to apply a reversed bias for erasing. Inaddition, the middle metal layer makes it technically difficult topattern the metal layer for each memory cell when the cells are verysmall.

The diffusion or drift of Cu-ions into semiconductor materials, likesilicon, is a well-known and troublesome phenomenon that has an adverseeffect on semiconductor devices (16). Generally a diffusion barrierlayer is added to prevent Cu metallization (17). Electrical-addressablenonvolatile memory devices have attracted considerable attention inrecent years due to their application in information technology. Siliconbased floating-gate memory (18), with a response time in thesub-millisecond, has played an important role in the modern electronicdevices, such as digital cameras. However, there is always a strongdemand for electronic nonvolatile memory devices that are less expensiveand better. Organic electrical bistable devices are promising in thisregard.

Organic electrical bistable devices with anorganic/metal-nanocluster/organic tri-layer structure sandwiched betweentwo electrodes have been made (19). These sandwich structures shownonvolatile memory behavior. Many other methods have also been reportedfor nonvolatile memory, such as phase change memory (20), programmablemetallization cell (21), nano-crystal memory (22), organic memory basedon scanning probe microscope (23), and organic memory in charge-transfercomplex system (6), polystyrene films (24), and molecular devices (25).

In view of the above, there is a continuing need to provide new andimproved electrically bistable structures which may be used inelectronic devices, such as memory devices and switches.

SUMMARY OF THE INVENTION

In accordance with the present invention, bistable electrical devicesare provided that are convertible between a low resistance (impedance)state and a high resistance (impedance) state. The bistable electricaldevices are well suited for use as electrical switching and memorydevices. In the present invention, we provide a new kind of organicbistable device (OBD) that utilizes a nano-surface (also referred to asa “buffer layer”) located on at least one of the electrodes. The OBD'sin accordance with the present invention provide high memory performancewithout any of the above-mentioned technical difficulties for memoryapplications.

The organic bistable electrical devices of the present inventiongenerally include a first electrode that has a first electrode surface.A layer of low conductivity organic material having a first surface anda second surface is provided wherein the first surface of the organiclayer is in electrical contact with the first electrode surface. Asecond electrode is provided that includes a second electrode surface.As a feature of the invention, a buffer layer is located between thesecond electrode surface and the second surface of organic layer. Thebuffer layer includes particles in the form of flakes or dots of a lowconducting material or insulating material that are present in asufficient amount to only partially cover the second electrode surface.The buffer layer controls metal ion migration from the electrode andprovides for the conversion of the bistable electrical device betweenthe low resistance (“on”) state and the high resistance (“off”) statewhen an electrical voltage is applied between the first and secondelectrodes.

The present invention utilizes one or more buffer layers to control themetal ion concentration within the organic layer interposed between twometal electrodes and provide electrical programmable nonvolatile memorydevices. Advantages of the memory devices of the present inventioninclude: 1) the memory devices have no conducting layer in between thetop and bottom electrodes. Therefore, it is not necessary to pattern theactive layer (which is composed of one or more buffer-layers and organiclayers) when making x-y memory-cell array type memory devices; 2) thewrite-read-erase voltage pulse can be the same direction, which isconvenient in an x-y electrical-addressable memory array device. This isbecause in x-y array type devices, a diode must be series connected witheach memory cell to prevent the sneak current. In addition, the on-statecurrent is much higher, at 0.1 V bias, the on-state current can go to 2A/cm². Both the On-state and Off-state are quite stable. As a result,this device is ideal for x-y array type memory and switch application.

The organic bistable electrical devices may be used to form a widevariety of memory devices wherein a memory input element is provided forapplying voltage to the organic bistable device to convert the activelayer between the low electrical resistance (high conductance) state andthe high electrical resistance (low conductance) state. The memorydevice further includes a memory read-out element which provides anindication of whether the bistable body is in the low or high electricalresistance state. As a feature of the present invention, the memoryread-out element may be a light-emitting diode which provides a visualindication of the electrical resistance state of the bistable body.

The above discussed and many other features and attendant advantages ofthe present invention will become better understood by reference to thedetailed description when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a diagrammatic representation of a bistable electricaldevice in accordance with the present invention that utilizes one bufferlayer. FIG. 1( b) is a diagrammatic representation of a bistableelectrical device in accordance with the present invention that utilizestwo buffer layers. FIG. 1( c) is a diagrammatic representation of abistable electrical device in accordance with the present invention thatutilizes three buffer layers.

FIG. 2 is a graph of the typical I-V characteristics of aCu/LiF/AIDCN/Cu device in accordance with the present invention that isinitially in the Off-state. The switching-on voltage ranges from V_(c1)to V_(c2). The switching-off voltage is higher than V_(c2). The readvoltage is the less than V_(c1).

FIG. 3 is a graph of the I-V characteristics of On-state and Off-statefor devices in accordance with the present invention. The readingvoltage is 0.2 V and the On/Off ratio is about 7 orders in magnitude.

FIG. 4( a) is a scanning tunneling microscope image of the surface of a10 μm×10 μLiF buffer layer (2.5 nm thickness). FIG. 4( b) is a scanningtunneling microscope image of the surface of a 1 μm×1 μm copper layer(100 nm thickness).

FIG. 5 is a schematic diagram of the measurement system used to measuretransient responses of exemplary Cu-OBD's in accordance with the presentinvention.

FIG. 6( a) is a graph of the transient response of an exemplary Cu-OBDfrom the Off-state to On-state. The response of the device was measuredby using a 50 Ohm read resistor. The response time is about 28 ms. FIG.6( b) is a graph of the transient response of the On-state of anexemplary Cu-OBD. The applied voltage pulse was 0.38V and 100 ns. Thedevice response was measured using a 50 Ohm resistor.

FIG. 7( a) is a graph of the transition speed of an exemplary Cu-OBDfrom the low resistance state to the high resistance state. The “appliedpulsed” curve represents the applied voltage pulse. The “deviceresponse” curve stands for the device response from On-state toOff-state by a 50 Ohm read resistor. The two current peaks are caused bythe capacitor effect (charging and discharging) of the off-state device.

FIG. 7( b) is a graph of the dynamic response of an exemplary Cu-OBD(initially at On-state) to an applied sharp voltage pulse (3.5 V peak 20ns half-height width). The response of the Cu-OBD was measured using 50Ohm read resistor. The negative peak indicates that the device alreadychanged to the off state. The transition process is so fast that itcould not be recognized by our current measurement systems.

FIG. 8 is a graph of the transient response of both the On-state andOff-state of an exemplary Cu-OBD. The On-state response follows theapplied pulse shape, while the Off-state shows a discharging effectwhich can be used to determine the device status.

FIG. 9( a) is a graph of the frequency dependence of capacitance of anexemplary Cu-OBD in the On-and-Off states. FIG. 9( b) is a graph of thefrequency dependence of the phase angle of impedance of an exemplaryCu-OBD in the On-and-Off states.

FIG. 10 is a schematic diagram of the circuit model used for calculatingthe devices' capacitance. The capacitance of the measurement system isless than 0.01 pF and is therefore omitted.

FIG. 11 is a graph of the frequency-dependence of the imaginary part ofthe impedance for the On-state of an exemplary Cu-OBD. The line in thegraph is the fitting results using formula (3) to fit the experimentaldata. The device's capacitance was determined to be about 0.3 pF whichis the same as the direct measurement shown in FIG. 9( a).

FIG. 12 is a graph of the frequency-dependence of the imaginary part ofthe impedance for the Off-state of an exemplary Cu-OBD. The line in thegraph is the fitting results using formula (4) to fit the experimentaldata. The device's capacitance was determined to be about 116 pF whichis the same as the direct measurement shown in FIG. 9( a).

FIG. 13( a) is a schematic diagram of an equivalent circuit for theoff-state exemplary Cu-OBD (pure capacitor model). FIG. 13( b) is aschematic diagram of an equivalent circuit for the on-state exemplaryCu-OBD from conducting filament formation. The resistor in FIG. 13( b)is the filament's resistance.

FIG. 14 is a graph of the frequency-dependence capacitance of anOff-state exemplary Cu-OBD with a resistor (mimics conducting filaments)parallel connected to the Cu-OBD. The capacitances with different valuesof parallel-connected resistors are the same as single Off-stateCu-OBD's (about 100 pF). This demonstrates the On-state of the exemplaryCu-OBD is not a result of filament formation.

FIG. 15( a) is a graph of the on-state I-V characteristics for anexemplary Cu-OBD with various active layer areas. The bold arrowrepresents an increase in area. The “+” line is 2 mm²; the “solidtriangle” line is 1 mm²; the “open square” line is 0.5 mm²; and the“solid square” line is 0.25 mm². FIG. 15( b) is a graph of thearea-dependence of the on-state current at 0.2 V bias.

FIG. 16 is graph of the I-V characteristics of two exemplary Cu-OBD'swith the same LiF layer thickness (2.5 nm), but different organic-layerthickness (45 nm, closed circles, and 100 nm, open circles,respectively). The On/Off ratios for the thicker and thinner devices are10⁸ and 10³, respectively.

FIG. 17( a) is a graph of the I-V behavior of an exemplary Cu-OBD at 80,160, 220, 250 and 300° K. When the temperature below 250 K the deviceexhibit non-linear I-V characteristics. Below the switching bias voltage(about 0.92 V), the none-linear I-V curves at the different temperatureoverlap. The switching voltage is the same at 250 and 300 K. FIG. 17( b)is a graph of the On-state I-V curves of an exemplary Cu-OBD at 80, 250and 300 K. The measurement sequence is, first 300° K, then cool down to80 K, then heating to 250 K.

FIG. 18 is a graph of the cycles test for an exemplary Cu-OBD. At first,the device was in the On-state. An erase voltage pulse was applied sothat the device changed to Off-state. The data shown graphically depictsa number cycles between On-and-Off states with the current measured at0.2 V bias. The stability of the On-state was tested by leaving thedevice alone for increasing amounts of time (such as 2 hours, 2 days)without any bias, and then measuring current through the device. Thedevice still remained at the On-state as shown in FIG. 18. The On-statecan be erased to the Off-state for continued cycles test. The Off-stateof the exemplary Cu-OBD was also stable.

FIG. 19 is a graph of the cycles test for an exemplary Cu-OBD(Cu/LiF(2.5 nm)/AIDCN(45 nm)/Cu). A 3V voltage pulse was used for“switch-off” and a 1.2 V voltage pulse was used for “switch-on”. Thecurrent was read at 0.2 V bias.

FIG. 20 is a graph of the heating-treatment and cycles test for anexemplary Cu-OBD. The Off-state current decreases (about 2 orders inmagnitude) after heating treatment. The heating treatment has no effecton the On-state current of the device. This heating effect of theOff-state current can only be observed for devices with relativelythinner AIDCN layers in which the Off-state current is relatively high.This is another method for decreasing the Off-state current of Cu-OBD's.

FIG. 21 (a) is a graph of the I-V characteristics of an exemplary Cu-OBDfor write-read-erase real time dynamic cycles test. FIG. 21( b) is agraph of the real time dynamic Write-Read-Erase cycle test of anexemplary Cu-OBD.

FIG. 22( a) is a SIMS Cu⁺ depth profile of an exemplary Cu-OBD. FIG. 22(b) is a SIMS Cu depth profile of an exemplary Cu-OBD. The On-state iscaused by high Cu⁺ concentration within the organic layer.

DETAILED DESCRIPTION OF THE INVENTION

An organic bistable electrical device in accordance with the presentinvention is shown in FIG. 1. The device includes an organic layer 4that is sandwiched between a first electrode 5 and a second electrode 2.The organic layer 4 is shown in the form of a layer. However, it will beunderstood that the organic layer can be provided in any number ofdifferent shapes. Organic layers in the form of a thin layer or film arepreferred since fabrication techniques for forming thin films are wellknown.

The organic layer 4 includes a first surface that is in electricalcontact with the first electrode 5. The organic layer 4 includes asecond surface that is located on the other side of the organic layer 4and which is in electrical contact with the second electrode 2. Thesecond electrode 2 is typically located on an insulating substrate 1. Ifdesired, the substrate 1 can be either ridged or flexible and made fromeither organic or inorganic materials that are well-know for use asinsulating substrates in electronic devices.

In accordance with the present invention, a buffer layer 3 is providedbetween the second electrode 2 and the organic layer 4 to providecontrol of metal ion migration into the organic layer 4. The bufferlayer 3 on the anode side is used for a number of purposes. For example,the buffer layer 3 is used to control metal ion injection from the anodeby decreasing the metal ion injection barrier at a proper appliedvoltage pulse (V_(c1)<V<V_(c2)) condition to realize the switch-onprocess. Another purpose is to control metal ion injection from theanode by increasing the copper ion injection barrier at higher appliedvoltage pulse condition (V>V_(c2)) to realize the switch-off process.Another purpose is to control metal ion injection from the anode bykeeping the metal ion injection properties (either no injection forOff-state or injection for On-state) at a low applied voltage pulsecondition to realize the read process. If desired, the switch-offprocess can be defined as the writing-process, while the switch-onprocess can be defined as the erasing process.

The organic bistable electrical device (OBD) is typically connected toan electronic control unit via electrical connections to the electrodes(not shown). The control unit is capable of providing an electricalvoltage bias across the organic layer 4 via the two electrodes 2 and 5to convert the OBD between low resistance (On) and high resistance (Off)states. In addition, the control unit is capable of, among other things,measuring current to determine the electrical resistance of the OBD.

The materials for the electrodes 2 and 5 can be metals or conductingmaterials like indium tin oxide (ITO). Suitable metals for use as theelectrodes include copper (Cu), gold (Au), silver (Ag), aluminum (Al)and other metals that have relatively high diffusion coefficients in theorganic layer. Copper is a preferred electrode material with devicesutilizing at least one copper electrode being referred to as a “Cu-OBD”.Either electrode can be the anode provided that it is copper or asimilar metal as set forth above.

The materials for the buffer layer should be insulating or lowconducting materials. A variety of low conducting or insulatingmaterials may be used to form the particles (in the form of insulatingdots or flakes) that make up the buffer layer. For example, LiF, NaCland other compounds similar to LiF and NaCl may be used. Such compoundstypically form flakes. Metal oxides, such as aluminum oxide (Al₂O₃), maybe used. These compounds typically form dots. The thickness of thebuffer layer is preferably from 1 to 10 nm thick with 2-5 nm beingespecially preferred. The thickness of the buffer layer can be as greatas 50 nm, if desired.

The buffer layer is composed of small dots or flake-like deposits whichare important for the observed electrical bistable behavior. It ispreferred that the insulating dots or flakes substantially cover theelectrode surface. However, some open spaces should remain between thedots or flakes. FIG. 4( a) is an STM image of a 10×10 μm² section of abuffer layer which shows LiF flakes on a copper surface. FIG. 4( a)shows what is meant by “substantially” covering the electrode surface.The degree of surface coverage and size of the flakes may be varied fromwhat is shown in FIG. 4( a) provided that the desired propertiesprovided by the buffer layer are not destroyed. The degree of surfacecoverage and flake or dot size is related to the thickness of the bufferlayer. In general, the thicker the buffer layer, the larger the degreeof coverage, and the surface morphology may be varied too.

The materials for the organic layer are preferably small conjugated lowconductivity organic materials. Suitable low conductivity materialsinclude organic semiconductors. Exemplary organic semiconductors includesmall molecular organic materials such as2-amino-4,5-imidazoledicarbonitrile (AIDCN);tris-8-(hydroxyquinoline)aluminum (Alq);7,7,8,8-tetracyanoquinodimethane (TCNQ); 3-amino-5-hydroxypyrazole(AHP); tris-(8-hydroxyquinolinolato)aluminum (Alq3); and copper or zinc2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc or ZnPc). Ifdesired, inorganic materials like silicon, gallium, gallium nitride andsimilar semi-conductors may be used in place of the organic layer. Theso-called “organic layer” is typically from 10 to 1000 nm thick.

The various electrodes, organic layers and buffer layers that make upthe organic bistable devices of the present invention can be fabricatedby vacuum thermal evaporation methods, spin-coating orcontinuous-coating techniques which are all well-known in the electronicdevice manufacturing field.

A second example of an OBD device, according to an embodiment of thecurrent invention, is shown schematically in FIG. 1( b) wherein bufferlayers 3 are provided between the organic layer 4 and both electrodes 2and 5. In FIG. 1(c), a third alternate embodiment is shown wherein twoorganic layers 4 are sandwiched between electrodes 2 and 5. Three bufferlayers 3 are used to separate the two organic layers 4 from each otherand to separate the electrodes 2 and 5 from the organic layers. As isapparent, a number of different combinations of organic layers withbuffer layers and electrodes are possible.

Examples of practice are as follows:

In the following examples, a number of OBD's were made and tested. Thebasic structure of the exemplary devices is shown in FIG. 1. Cu wasselected for the electrodes due to its high diffusion coefficient (25).The buffer layers were approximately 4 μm thick (unless otherwise noted)and included dielectric materials, such as lithium fluoride (LiF) andaluminum oxide. Materials with low conductivity, good film formabilityand stability such as 2-amino-4,5-imidazoledicarbonitrile (AIDCN),tris-8-(hydroxyquinoline)aluminum (Alq₃), and Zinc2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (ZnPc) were selectedfor the organic layers.

The OBD's of the present invention can be fabricated by simple vacuumthermal evaporation methods, spin-coating or continuous-coatingtechniques. The Cu-OBD's in these examples were fabricated by vacuumthermal evaporation methods. All the depositions were performed in ahigh vacuum about 1×10⁻⁶ torr. A preferred process includes depositingall films for device fabrication without breaking the vacuum.

The buffer layer controls Cu ions injection into the organic layer atvarious applied voltages. At a low critical voltage pulse V_(c1)(generally, ranging from 0.2 to 3 V) it allows Cu ions injection fromthe anode into the organic layer, which switches the device to highconductance state (On-state), while above a relative high criticalvoltage V_(c2) (generally above 3 V in 10 nanoseconds width) it can shutdown Cu ions injection and restore the device to low conductance state(Off-state). The two states differ in their electrical conductivity byseveral orders (3-9) of magnitude depending on the device fabricationprocessing, and can be precisely switched by controlling the Cu⁺concentration through the application of external voltage pulses. Asmall voltage pulse (less than 0.1V, ten nanoseconds width) can be usedto read. At no bias condition both On-state and Off-state are quitestable even being heated to 110° C., which makes it suitable fornonvolatile memory application. The On-state current density of thedevice is quite high (˜2A/cm² @ 0.1V bias). The devices are especiallywell suited for flash memory applications and for driving light-emittingpixels in display applications.

FIG. 2 shows the typical I-V characteristics of the Cu-OBDs when thedevice is initially in the Off-state (high impedance), the highconductive state can be excited by a voltage V (V_(c1)<V<V_(c2))).Higher voltage (V>V_(c2)) can restore the device to the Off-state. TheOff-state is stable if the following applied voltage is less thanV_(c1). Once the device is excited by the small voltage to the On-state,it will remain at the On-state for prolonged periods of time if thefollowing applied voltage is less than V_(c2). Therefore, we can usevery small voltage (0.1 or 0.2V) to read the On-state and Off-state. TheOn-Off ratio of the devices can be as high as 7 orders in magnitude (SeeFIG. 3).

It is believed that the OBD devices operate according to the followingprinciples. Copper that diffuses into other materials is in a positivelycharged state (26), and the copper ions drift in both silicon (27) andorganic materials (28), and cause copper metallization. Generallydiffusion barrier layers are used to prevent this metallization (29).The diffusion barrier provides an interface adhesion (or an energybarrier) to prevent Cu⁺ diffusion and metallization (30). For Cu-OBD's,when positive bias is applied, copper is ionized at the inner-face ofthe anode and acts as the Cu⁺ source. When the energy of Cu⁺ is highenough (larger than eV_(c1)) to overcome the energy barrier, they areinjected into the organic layer, and drift towards the cathode. When theCu⁺ ions reach the cathode, a continuous Cu⁺ distribution within theorganic layer is established where the organic layer is metalized by theCu⁺ and exhibits the ON-state. This is also consistent with the delaytime during the switch-ON process as shown later in FIG. 7( a).Providing the delay time is solely caused by the Cu⁺ traveling timethrough the organic layer, one can estimate that the drift velocity ofCu⁺ in AIDCN film is about 5×10⁴ cm/s under the electric field of1.2×10⁵ V/cm, and the diffusivity of Cu⁺ in AIDCN film is about 10⁻¹⁰cm²/s at room temperature, which is smaller than in case of silicon(about 10⁻⁷ cm²/s) (31). By selecting organic materials with arelatively high Cu⁺ diffusion coefficient, faster switch-ON speeds canbe expected.

For Cu-OBDs, when the applied bias is over the second critical voltage(V_(c2)), it undergoes the switch-OFF process and the device changes tothe OFF-state (FIG. 2), which indicates that Cu⁺ injection is prohibitedwhile the residual Cu⁺ within the organic layer drift towards thecathode and is reduced to Cu. Once a gap larger than a percolationthreshold is formed within the organic layer where the Cu⁺ is free, thedevice will be switched OFF. The rest of Cu ions will continuously drifttowards the cathode until no Cu ions remain within the organic layer.Hence, the transition speed from On-state to Off-state is very fast. Onepossibility of no Cu⁺-injection at a bias voltage larger than V_(c2) maybe due to the dipole alignment of the buffer layer. Since high dipolemoment materials are used for the buffer layer, when the appliedelectric field is high enough, dipole alignment may happen (32), whichtremendously increases the energy barrier and prohibits Cu⁺ injection.The polarized dipoles may restore to a random orientation when the biasis removed (33), allowing the rewritable character of the devices.

The surfaces of the LiF buffer layer and Cu electrodes were investigatedby using a scanning tunneling microscope (STM). FIG. 4( a) shows the STMimage of an LiF layer with 2.5 nm in thickness on a pre-deposited Cusubstrate. It can be seen from FIG. 4( a) that the deposited LiF layerof dots is flake-like, which is important for the observed electricalbistable behavior. FIG. 4( b) shows the STM image of the surface of thedeposited Cu electrode layer. It can be seen from FIG. 4( b) that thesurface of the Cu layer is quite smooth compared with the LiF layer,which indicates that the surface structure of LiF shown in FIG. 4( a) iscaused by LiF itself and not the Cu surface morphology. It is preferredthat the insulating dot or flakes substantially cover the electrodesurface, but that some open spaces remain between the dots or flakes.FIG. 4( a) shows what is meant by “substantially” covering the electrodesurface. The degree of surface coverage may be varied from what is shownin FIG. 4( a) provided that the desired properties provided by thenano-layer are not destroyed. The thickness of the flake or dot layercan range from 1 to 50 nanometers. Thickness on the order of 2 to 5nanometers is preferred.

The transition speed of the Cu-OBDs from both the high-resistance stateto low resistance state and from the low-resistance state to thehigh-resistance state is measured by transient measurement. Themeasurement setup is shown in FIG. 5. The transition speed of theOn-to-Off state is relatively slow, about several ten milliseconds. FIG.6( a) shows the transient response of a Cu-OBD from the Off-state to theOn-state. It can be seen from FIG. 6( a) that when the voltage pulse isapplied to the device, the device initially keeps its high-resistancestate. After about a 28 ms delay time, the device jumps tolow-resistance state. The real transition speed from high to lowresistance state is quite small as shown by in FIG. 6( a). The tens ofmilliseconds delay time indicates that Cu⁺ ions travel from the anode tothe cathode. The electrical behavior of the On-state Cu-OBD is like apure resistor. The transient electrical behavior of the On-state Cu-OBDis shown in FIG. 6( b). It can be seen from FIG. 6( b) that no capacitoreffects (charging and discharging) are observed for the on-stateCu-OBDs. The current follows the applied voltage pulse, which indicatesthat the on-state Cu-OBDs exhibits pure resistor behavior. Thisconclusion is confirmed by the impedance measurements set forth below.

The transition speed from the On-to-Off state of the Cu-OBDs was shownto be quite fast. It is less than 10 ns, which is within the limitationof our measurement system. By applying a relatively high voltage pulse(about 3 volts) to the device, the device can change its state from lowresistance to high resistance in less than nanoseconds. FIG. 7( a) showsthe transient response of an exemplary Cu-OBD (initially at the lowresistance state) to an applied voltage pulse. Before doing thistransition speed measurement, DC I-V curve measurements were taken tomake sure the device was initially at the low resistance state. Thetransition speed from low resistance state to high resistance state isless than nanoseconds. Therefore, a narrow voltage pulse can be used toexcite the devices from the on state to the off state. FIG. 7( b) showsthe dynamic response of Cu-OBDs (initially at On-state) to a very sharpvoltage pulse.

The transient response of On-state and Off-state of Cu-OBDs to a verysharp applied voltage pulse is quite different as shown in FIG. 8. Itcan be seen from FIG. 8 that the current response of the On-state devicefollows the applied voltage pulse very well, while a negative peak canbe seen clearly in the Off-state device response. This is the capacitordischarging effect of the Off-state devices. Therefore, very shortvoltage pulses (less than 20 ns) can be used for reading. The readingtime of a typical Cu-OBD can be less than 20 ns. By further decreasingthe device area, the speed of the device will be much faster as thecapacitance of the device decreases.

Impedance measurements were carried out using an HP 4284A LCR meter. Thefrequency dependence of the device's capacitance is shown in FIG. 9( a).If can be seen from FIG. 9( a) that the Off-state devices' capacitanceis about 100 pF, while the On-state devices' capacitance is about 0.3pF. The capacitance decreased more than 2 orders in magnitude after thedevice was switched from Off-state to On-state.

The phase of the impedance for Cu-OBD at both the On-state and theOff-state are shown in FIG. 9( b). It can be seen from FIG. 9( b) thatthe phase is nearly zero for the device in the On-state indicating apure resistor case. The phase for the device at the Off-state is nearly−90°, indicating a pure capacitor case. The capacitance data shown inFIG. 9( a) is directly measured by a CPCR mode of the LCR meter. Toconfirm this data, frequency-dependence impedance data were alsomeasured, from which capacitance can be calculated by a circuit modelshown in FIG. 10. The impedance of this circuit is

$\begin{matrix}{Z = {{Rs} + \frac{r}{1 + {r^{2}\omega^{2}c^{2}}} - {{\mathbb{i}}{\frac{\omega\;{cr}^{2}}{1 + {r^{2}\omega^{2}c^{2}}}.}}}} & (1)\end{matrix}$

When 1/(ωc)>>r, for Cu-OBDs at On-state case,

$\begin{matrix}{Z = {R + \frac{r}{1 + {r^{2}\omega^{2}c^{2}}} - {{\mathbb{i}}\; r^{2}\omega\;{c.}}}} & (2)\end{matrix}$

Therefore, for On-state Cu-OBDs, the imaginary part of impedance isproportional to the frequency f (Hz):Z _(o) sin(θ)=−2πr ² cf.  (3)Here, Z_(o) is the amplitude of impedance.

When 1/(ωc)<<r, for the Off-state of Cu-OBD case, the imaginary part ofimpedance is proportional to 1/f,Z _(o) sin(θ)=−1/(2πcf).  (4)

FIG. 11 shows the frequency-dependence of the imaginary part of theimpedance of the On-state CU-OBD. Using formula (3) to fit theexperimental data, the device's capacitance was determined to be about0.3 pF.

Using formula (4) to fit the imaginary part of impedance for Off-stateCu-OBD, the Off-state capacitance of the device can be obtained. FIG. 12shows the frequency-dependence of the imaginary part of the impedance ofthe Off-state Cu-OBD. The device's capacitance was determined to beabout 116 pF which is the same as the direct measurement shown in FIG.9( a).

It apparent from the above that an Off-state Cu-OBD behaves as a purecapacitor. If the On-state of Cu-OBDs is caused by conducting filamentformation, the area of the filaments should be much smaller than thedevice's area. Generally the diameter of the filaments is in themicrometer range and has a certain resistance. Therefore, the formationof conducting filaments in the device should not change the capacitanceof the device. Instead, it is equivalent to a resistor that is parallelconnected to the device's capacitance. FIG. 13 shows the equivalentcircuit of the On-state from a conducting filament formation point ofview.

A resistor was parallel connected to an Off-state Cu-OBD, by changingthe resistance of the resistor from 160 Ohm to 100 kOhm to mimic thepossible resistance of the conducting filament. The capacitance of thedevice was then measured. As expected, the paralleled resistor (theformation of conducting filament) doesn't change the capacitance of thedevice. FIG. 14 shows the frequency-dependence capacitance of anOff-state Cu-OBD with a resistor parallel connected to it. Thecapacitances with different values of parallel-connected resistors arethe same as single Off-state Cu-OBDs (about 100 pF). As shown in FIG. 9(a) and FIG. 11, the capacitance of the On-state Cu-OBD is much smallerthan the Off-state Cu-OBD (more than two orders). Therefore, the sectionof conducting path for On-state device is the same as the device's area.This would not be the case for filament formation for On-state Cu-OBDs.Generally, the On-state current has little, if any, relation with thedevice's area if conducting filament formation is involved. The On-stateI-V characteristics for Cu-OBDs with various devices' area are shown inFIG. 15( a). The area-dependent of On-state current at 0.2 V bias isshown in FIG. 15( b). The On-state current at the same bias is nearlyproportional to the devices' surface area.

The On/Off ratio is an important factor for device's application. TheOff-state current of the devices may not be low enough. Therefore,determining how to decrease the leakage current and increase the On/Offration is very important. By changing the thickness of the buffer layer(LiF) and the organic layer (AIDCN). It was found that about 2.5 nm forLiF layer and about 100 nm for the AIDCN layer is the preferredcondition to obtain the highest On/Off ratio. Up to now, a 10⁸ On/Offratio has been achieved for Cu-OBDs. By decreasing the thickness of theorganic layer, the Off-state current will go up, leading to a decreasein the On/Off ratio. FIG. 16 shows the I-V characteristics of twoCu-OBDs with the same LiF layer thickness (2.5 nm) but different Organiclayer thickness (45 and 100 nm respectively). The opened circlesrepresent the data for the Cu-OBD with a thicker AIDCN layer (100 nm).For this device, the On/Off ration can reach as high as 10⁸. In fact,the Off-state current is within the limitation of the measurementsystem. The closed circles stand for the data for the Cu-OBD with athinner AIDCN layer (45 nm). For this device, the On-state current is alittle higher, but the Off-state current is much larger than the thickerone. The On/Off ratio for the Cu-OBD with 45 nm-thickness AIDCN layer isjust above 10³.

To investigate the low temperature behavior of the exemplary Cu-OBD's, aPDF-475 dewar was used to study the I-V behavior from 80° K to 300° K.It was found that below 250° K, the devices are difficult to betriggered from the Off-state to the On-state. FIG. 17( a) shows the I-Vbehavior of a Cu-OBD at 80, 160, 220, 250 and 300° K. When thetemperature below 250° K, the device exhibits non-linear I-V behavior.Below the switching bias voltage (about 0.92 V), the none-linear I-Vcurves at the different temperature overlapped. At temperature above250° K the devices can be switched between On-Off states. The switchingvoltage is the same at 250 and 300° K

The On-state I-V curves at 80, 250 and 300° K are shown in FIG. 17( b).First the On-state I-V curve at 300° K was measured, then the device wascooled down to 80° K, where the device remained in the On-state. Aftermeasuring the On-state I-V characteristics at 80° K, the device wasswitched to the Off-state by applying a 4V voltage pulse. After beingheated to 250° K, the device again was switched to the On-state byapplying a voltage pulse (1V). Then the On-state I-V characteristics at250° K were measured. It can be seen from FIG. 17( b) that there is somethermal hysteretic behavior. The On-state of CU-OBDs has a linear I-Vrelation, indicating that the charge transport in the On-state of theCu-OBD is not a hopping process. Although the Off-state I-Vcharacteristics are non-linear, they are weakly temperature-dependent.Before a switch-on voltage is applied, the non-linear I-V behavior istemperature-independent. At 2 V bias, the Off-state current increasesonly about 15% when the temperature of the device increased from 80 to220° K If charge transport is a thermal hopping process, then theactivation energy, E_(a) (I=I_(o) exp(−E_(a)/(kT)) is calculated to be1.6 meV, which is quite small, even compared with 80 K (6.9 meV).

The exemplary Cu-OBD's that were prepared were found to be non-volatilerewritable memory devices. Once a Cu-OBD is switched to either state, itremains at that state without any bias applied for a long time (morethan months). In write-read-erase-read (WRER) cycles test, a 3 V voltagepulse was used for erase, a 1.2 V voltage pulse for write, and a 0.2 Vvoltage bias for reading. FIG. 18 shows the cycles test for an exemplaryCu-OBD. It should be noted that this cycles test is not a time-dependentdynamic cycles test. The real-time dynamic cycles test will be set forthbelow. At first, the device was in the On-state with an erase voltagepulse being applied to change the device to the Off-state. The datashown on FIG. 18 is cycle-number dependent On-and-Off states current at0.2 V. The stability of the On-state was tested by leaving it alone forsome time (such as 2 hours, 2 days) without any bias, then measuring itagain. It still remained at the On-state as shown in FIG. 18. TheOn-state certainly can be erased to the Off-state for continued cyclestest. The Off-state of Cu-OBDs is also stable. When the device wasswitched to the Off-state and kept in vacuum chamber for 37 days, it wasstill at the Off-state and could be switched to the On-state for furthercontinuous cycles testing (see the last three dots at the right side ofFIG. 18).

FIG. 19 shows another Cu-OBD (Cu/LiF(2.5 nm)/AIDCN(45 nm)/Cu) cyclestest. 3V was used for erase, 1.2 V for write, and the current was readat 0.2 V bias. It can be seen from FIG. 19 that the On-state current isnearly the same during the cycles test, but the Off-state current showsdecreasing tendency during cycles test.

The above stability tests were performed at room temperature. A furtherdemonstration of the properties of devices in accordance with thepresent invention involved heating the device and checking the device'sstate (On, or Off state) before and after heating treatment. FIG. 20shows the heating-treatment and cycles test. First, the device was atOn-state, as shown by the first dot in FIG. 20, then it was heated to110° C. for 1 minute. After this heating treatment, it was still at theOn-state as shown by the second dot in FIG. 20. Then, a 3 V voltagepulse was applied to restore it to the Off-state. The third dot in FIG.20 shows that the device was successfully restored to the Off-state thatwas then switched to On-state again by applying a 1.2 V voltage pulse asconfirmed by the fourth dot in FIG. 20. The device was heated again at110° C. for 2 minutes, 10 minutes, and even 1 hour. The On-state stillremained and could be restored to the Off-state after applying an erasevoltage pulse. The stability of Off-state of Cu-OBDs is also stableduring and after heating treatment as indicated by dot Nos. 6-7 and10-11.

It can be seen from FIG. 20 that the Off-state current decreasessubstantially (about 2 order in magnitude) after heating treatment,while heating treatment has no effect on the On-state current of thedevice. This Heating effect of the Off-state current can only beobserved for devices with relatively thinner AIDCN layer in which theOff-state current is relatively high. This is another method fordecreasing the Off-state current of Cu-OBD's.

A Keithley 2400 was used to apply programmable voltage pulses in orderto conduct WRER cycles tests. The typical WRER cycles cell are shown inFIGS. 21( a) and (b). FIG. 21( a) is the I-V characteristics for thecycles-test Cu-OBD. FIG. 21( b) is the real time dynamic cycles test.

It is believed that in accordance with the present invention, theOn-state and Off-states are due to the Cu⁺ distribution and subsequentmetallization and de-metallization or the organic layer as controlled bythe buffer layer. This belief is supported by the secondary ion massspectrometry (SIMS) depth profile measurement for Cu⁺ ion and Cu atom inexemplary devices in both states. It was found that Cu⁺ ion are driveninto the organic layer in the On-state (metallization process), whileCu⁺ ions drifted out of the organic layer in the Off-state as shown inFIG. 22( a) (de-metallization process). Therefore, the ON-and-OFF statescan be switched back and forth by controlling the Cu⁺ ion distributionprofile within the organic layer. The atomic Cu distribution in theCu-OBDs (FIG. 22( b)) was found to be low within the organic layer forboth of the states. Accordingly, the dynamic Cu⁺ concentration withinthe organic layer is believed to be responsible for the observedbistability of Cu-OBDs.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention. Thebuffer layers (nanosurfaces) of the present invention may be used in awide variety of bistable devices as an interface between the electrodeand the organic bistable layer. For example, the active layer of thepresent OBD's (organic layer plus one or more buffer layers) may be usedto replace the bistable bodies in devices of the type described in PCTApplication No. US01/17206. Accordingly, the present invention is notlimited to the above preferred embodiments and examples, but is onlylimited by the following claims.

BIBLIOGRAPHY

-   1. J. F. Dewald, A. D. Pearson, W. R. Northover, and W. F. Peck,    Jr., J. Electrochem. Soc., 109, 243c (1962). “Semi-conducting    glasses”-   2. Ovshinsky, S. R. Localized states in the gap of amorphous    semiconductors. Phys. Rev. Lett., vol. 36 (no. 24), 14 Jun. 1976, p.    1469-72.-   3. Yu, G. Kriger, N. F. Yudanov, I. K., Igumenov, and S. B.    Vashchenko, J. Struct. Chem., 34 (1993). “Study of test structures    of a molecular memory element”-   4. H. J. Hovel and J. J. Urgell, J. Appl. Phys. 42, 5076 (1971).    “Switching and memory characteristics of ZnSe—Ge heterojunctions”-   5. R. Kumai, Y. Olimoto and Y. Tokura, Science, 284, 1645 (1999).    “Current-induced insulator-metal transition and pattern formation in    an organic charge-transfer complex”.-   6. R. S. Potember, T. O. Poehler and D. O. Cowan, Appl. Phys. Lett.    34, 407 (1979). “Electrical switching and memory phenomena in    Cu-TCNQ thin films”-   7. F. Garnier, R. Hajlaoui, A. Yassar, and P. Shirakawa, Science    265, 1684 (1994)-   8. F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. A.    Andersson, Q. Pei, and A. J. Heeger, Science 273, 1883 (1997).-   9. R. Kumai, Y. Okimoto, Y. Tokura, Science 284, 1645 (1999).-   10. W. Fujita, and K. Awaga, Science 286, 261 (1999).-   11. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.    Mackay, R. H. Friend, P. L. Burn, and A. B. Holmes, Nature, 347, 539    (1990).-   12. T. Yamada, D. Zou, H. Jeong, Y. Akaki, and T. Tsutsui, Synthetic    Metals, 111-112, 237 (2000).-   13. J. Liu, Y. Shi, L. P. Ma, and Y. Yang, J. Appl. Phys. 88, 605    (2000).-   14. Y. Hamada, C. Adachi, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys.    31, 1812 (1992).-   15. Y. Yang et al., U.S. Pat. No. 5,563,424, Oct. 8, 1996.-   16. McBrayer, J. D., Swanson, R. M., and Sigmon, T. W., Diffusion of    metals in silicon dioxide. J. Electrochem. Soc. 133, 1242(1986).-   17. Rosenberg, R., Edelstein, D. C., Hu, C. K., Rodbell, K. P.,    Copper metallization for high performance silicon technology, Annual    Rev. Materials Science, 30, 229 (2000). Kaloyeros, A. E.,    Eisenbraun, E., Ultrathin diffusion barriers/liners for gigascale    copper metallization, Annual Rev. Materials Science, 30, 363(2000).-   18. W. D. Brown, J. E. Brewer, Nonvolatile semiconductor memory    technology, IEEE Press, New York, (1998).-   19. Ma, L. P., Pyo, S. M., Xu, Q. F. and Yang, Y., Nonvolatile    electrical bistability of organic/metal-nanocluster/organic system,    Appl. Phys. Lett. 82, 1419(2003), Ma, L. P., Liu, J, and Yang, Y.,    organic electrical bistable devices and rewritable memory cells,    Appl. Phys. Lett. 80, 2997 (2002), Ma, L. P. Liu, J., Pyo, S. M. and    Yang, Y., Organic bistable light-emitting devices, Appl. Phys. Lett.    80, 362(2002), Ma, L. P., Liu, J., Pyo, S. M., Xu, Q. F. and Yang,    Y., Organic bistable devices, Mol. Cryst. Liq. Cryst. 378,    185(2002).-   20. Nakayama, K., Kojima, K., Imai, Y., Kasai, T., Fukushima, S.,    Kitagawa, A., Kumeda, M., Kakimoto, Y., Suzuki, M., Nonvolatile    memory based on phase change in Se—Sb—Te glass, J. J. Appl. Phys.,    Part 1, 42 (2A), 404(2003).-   21. Kozicki, M. N. and West, W. C., Programmable metallization cell    structure and method of making same, U.S. Pat. No. 5,761,115 (1998).-   22. Ostraat, M. L., De Blauwe, J. W., Green, M. L., Bell, L. D.,    Brongerma, M. L., Casperson, J. R., Flagan, C. and Atwater, H. A.,    Synthesis and characterization of aerosol silicon nanocrystal    nonvolatile floating-gate memory devices, Appl. Phys. Lett. 79,    433(2001).-   23. Ma, L. P., Yang, W. J., Xie, S. S. and Pang, S. J., Ultrahigh    density data storage from local polymerization by a scanning    tunneling microscope, Appl. Phys. Lett. 73, 3303(1998).-   24. Segui, Y., Bui Ai, and Carchano, H., Switching in polystyrene    films: Transition from on to off state, J. Appl. Phys. 47,    140(1976).-   25. J. Chen, W. Wang, M. A. Reed, A. M. Rawlett, D. W. Price,    and J. M. Tour, Room-temperature negative differential resistance in    nanoscale molecular junctions, Appl. Phys. Lett. 77, 1224(2000).-   26. Beeler, F., Andersen, O. K, and Scheffler, M., Theoretical    Evidence for Low-Spin Ground States of Early Interstitial and Late    Substitutional 3d Transition-Metal Ions in Silicon, Phys. Rew. Lett.    55, 1498(1985).-   27. Istratov, A. A., Weber, E. R, Physics of copper in silicon, J.    Electrochemical Society, 149, G21(2002). Istratov, A. A., Flink, C.,    Hieslmair, H., McHugo, S. A., Weber, E. R., Diffusion, solubility    and gettering of copper in silicon, Materials Science. and    Engineering Technology B, 72, 99(2000). Lee, K. L., Hu, C. K, Tu, K.    N., In-situ scanning electron microscope comparison studies on    electromigration of Cu and Cu(Sn) alloys for advanced chip    interconnects, J. Appl. Phys. 78, 4428-4437(1995).-   28. Loke, A. L. S., Wetzel, J. T., Townsend, P. H., Tanabe, T.,    Vrtis, R. N., Zussman, M. P., Kumar, D., Ryu, C., Wong, S. S.,    Kinetics of copper drift in low-kappa polymer interlevel    dielectrics, IEEE Transactions on Electron Devices, 46, 2178(1999).-   29. Wang, M. T., Lin, Y. C., Chen, M. C., Barrier properties of very    thin Ta and TaN layers against copper diffusion, J. Electrochemical    Society, 145, 2538(1998). Faltermeier. C., Goldberg, C., Jones, M.,    Upham, A., Manger, D., Peterson, G., Lau, J., Kaloyeros, A. E.,    Arkles, B., Paranjpe, A., Barrier properties of titanium nitride    films grown by low temperature chemical vapor deposition from    titanium tetraiodide, J. Electrochemical Scoiety, 144, 1002(1997).    Krishnamoorthy, A., Chanda, K., Murarka, S. P., Ramanath G.,    Ryan, J. G., Self-assembled near-zero-thickness molecular layers as    diffusion barriers for Cu metallization, Appl. Phys. Lett. 78,    2467(2001).-   30. Lane, M. W., Liniger, E. G., and Lioyd, J. R., Relationship    between interfacial adhesion and electromigration in Cu    metallization, J. Appl. Phys. 93, 1417(2003).-   31. Istratov, A. A., Flink, C., Hieslmair, H., Weber, E. R., and    Heiser, T., Intrinsic diffusion coefficient of interstitial copper    in silicon, Phys. Rev. Lett. 81, 1243(1998).-   33. Sprang, H. A. van, and Venne, J. L. M. van de, Influence of the    surface interaction on threshold values in the cholestericnematic    phase transition, J. Appl. Phys. 57, 175(1985). Boyd, G. D., Cheng,    J, and Ngo, P. D. T., liquid-crystal orientational bistability and    nematic storage effects, Appl. Phys., Lett. 36, 556(1980), Gruler,    H., and Cheung, L., Dielectric alignment in an electrically    conducting nematic liquid crystal, J. Appl. Phys. 46, 5097(1975).    Patel, J. S., Room-temperature switching behavior of ferroelectric    liquid c2ystals in thin cells, Appl. Phys. Lett. 47, 1277(1985).-   33. Yang, K. H., Chieu, T. C., and Osofsky, S., Depolarization field    and ionic effects on the bistability of surface-stabilized    ferroelectric liquid-crystal devices, Appl. Phys. Lett. 55,    125(1989).

1. A bistable electrical device that is convertible between a lowresistance state and a high resistance state, said device comprising: afirst electrode that includes a first electrode surface; a layer of lowconductivity material having a first surface and a second surfacewherein said first surface of said layer of low conductivity material isin electrical contact with said first electrode surface; a secondelectrode that includes a second electrode surface; and a buffer layerlocated between said second electrode surface and the second surface ofsaid layer of low conductivity material, wherein said buffer layer isconstructed of a material and a form to provide control of metal ionmigration from said second electrode to said layer of low conductivitymaterial when a voltage is applied between said first and secondelectrodes to thereby provide a bistable behavior of said bistableelectrical device, and wherein said bistable electrical device isconvertible between a high electrical resistance state and a lowelectrical resistance state when a first voltage is applied between saidfirst and second electrodes and said bistable electrical device isconvertible between said low electrical resistance state and a highelectrical resistance state when a second voltage is applied betweensaid first and second electrodes, said second voltage having a greatermagnitude than said first voltage, wherein said organic low conductivitymaterial is 2-amino-4,5-imidazoledicarbonitrile.
 2. A bistableelectrical device that is convertible between a low resistance state anda high resistance state, said device comprising: a first electrode thatincludes a first electrode surface; a layer of low conductivity materialhaving a first surface and a second surface wherein said first surfaceof said layer of low conductivity material is in electrical contact withsaid first electrode surface; a second electrode that includes a secondelectrode surface; a buffer layer located between said second electrodesurface and the second surface of said layer of low conductivitymaterial; and a diode connected to at least one of said first or secondelectrodes, wherein said buffer layer is constructed of a material and aform to provide control of metal ion migration from said secondelectrode to said layer of low conductivity material when a voltage isapplied between said first and second electrodes to thereby provide abistable behavior of said bistable electrical device, and wherein saidbistable electrical device is convertible between a high electricalresistance state and a low electrical resistance state when a firstvoltage is applied between said first and second electrodes and saidbistable electrical device is convertible between said low electricalresistance state and a high electrical resistance state when a secondvoltage is applied between said first and second electrodes, said secondvoltage having a greater magnitude than said first voltage.
 3. A memorydevice comprising: A) a bistable electrical device comprising: a firstelectrode that includes a first electrode surface; a layer of lowconductivity material having a first surface and a second surfacewherein said first surface of said layer of low conductivity material isin electrical contact with said first electrode surface; a secondelectrode that includes a second electrode surface; and a buffer layerlocated between said second electrode surface and the second surface ofsaid layer of low conductivity material; B) a memory input element forapplying a voltage to said bistable electrical device to convert saidbistable electrical device between a low electrical resistance state anda high electrical resistance state; and C) a memory readout elementwhich provides an indication of whether said bistable electrical deviceis in said low electrical resistance state or said high electricalresistance state, wherein said buffer layer is constructed of a materialand a form to provide control of metal ion migration from said secondelectrode to said layer of low conductivity material when a voltage isapplied between said first and second electrodes to thereby provide abistable behavior of said bistable electrical device, and wherein saidmemory readout element is a light emitting diode.
 4. A bistableelectrical device that is convertible between a low resistance state anda high resistance state, said device comprising: a first electrode thatincludes a first electrode surface; a layer of low conductivity materialhaving a first surface and a second surface wherein said first surfaceof said layer of low conductivity material is in electrical contact withsaid first electrode surface; a second electrode that includes a secondelectrode surface; and a buffer layer located between said secondelectrode surface and the second surface of said layer of lowconductivity material, wherein said buffer layer is constructed of amaterial and a form to provide control of metal ion migration from saidsecond electrode to said layer of low conductivity material when avoltage is applied between said first and second electrodes to therebyprovide a bistable behavior of said bistable electrical device, whereinsaid bistable electrical device is convertible between a high electricalresistance state and a low electrical resistance state when a firstvoltage is applied between said first and second electrodes and saidbistable electrical device is convertible between said low electricalresistance state and a high electrical resistance state when a secondvoltage is applied between said first and second electrodes, said secondvoltage having a greater magnitude than said first voltage, wherein saidlayer of low conductivity material comprises an inorganic material, andwherein said inorganic material comprises a material selected from thegroup of materials consisting of silicon, gallium and gallium nitride.5. A bistable electrical device according to claim 2 wherein said diodeis a light emitting diode.
 6. A bistable electrical device according toclaim 2 or 3, wherein said layer of low conductivity material comprisesan inorganic material.
 7. A bistable electrical device according toclaim 2 or 4, wherein said second electrode consists essentially ofcopper.
 8. A bistable electrical device according to claim 1, 2 or 4wherein said second electrode comprises a metal selected from the groupof metals consisting of metals having a relatively high diffusioncoefficient in said layer of low conductivity material.
 9. A bistableelectrical device according to claim 1, 2 or 4, wherein said secondelectrode comprises a metal selected from the group of metals consistingof copper, gold, and silver.
 10. A bistable electrical device accordingto claim 1, 2, 3 or 4 wherein said buffer layer is from 1 nanometer to50 nanometers thick.
 11. A bistable electrical device according to claim1, 2, 3 or 4 wherein said buffer layer comprises particles of a lowconducting material or insulating material that substantially coverssaid second electrode surface.
 12. A bistable electrical deviceaccording to claim 1, 2, 3 or 4 wherein said buffer layer comprises alow conducting material or insulating material selected from the groupconsisting of sodium chloride, lithium fluoride and aluminum oxide. 13.A bistable electrical device according to claim 1, 2, 3 or 4 whereinsaid buffer layer is from 2 nanometers to 5 nanometers thick.
 14. Abistable electrical device according to claim 10 wherein said bufferlayer is from 1 nanometer to 10 nanometers thick.